Synthetic micro-porous polymer films have been increasingly developed for applications in tissue engineering, drug delivery, photovoltaics, low-k dielectrics and laser fusion targets. To produce micro-porosity in thermoset resins, polymerization-induced phase separation (PIPS) can be used. This approach consists of polymerizing a mixture of polymer precursors and a non-reactive solvent, termed the porogen, which templates porosity. The porogen must be a solvent for the precursor but a non-solvent for the polymer. In the PIPS process, phase separation (nucleation and coarsening) and polymerization occur simultaneously. Owing to the substantial decrease in the entropy of mixing on polymerization, the initially homogeneous solution separates into a solvent-rich phase and polymer-rich phase. When polymerization and phase separation are complete, solvent may be removed by drying, leaving behind a porous structure.

We have recently designed and built a unique axisymmetric initiated chemical vapor deposition (i-CVD) vacuum reactor. Using this system, several poly (methyl methacrylate) (PMMA) films were grown from the vapor phase at a rate of ~ 0.4 micron/hour. FT-IR analysis confirms the formation of PMMA functional groups during deposition, and the removal of unreacted monomer. Several reactor design parameters such as hot-zone temperature, reactor base-pressure, substrate temperature, and the feed component molar feed ratios were optimized to obtain higher deposition rates. Resulting as-deposited films have a smooth, featureless surface morphology with average RMS around 20 nm. Molecular weight was measured using size exclusion chromatography techniques, and polymer chains are about 20K Daltons. The molecular weight can be adjusted to some extent by changing the monomer/initiator molar feed ratio or by introducing a third component (n-butanol).

We are exploring new ways to engineer porosity into polymer films using solventless i-CVD techniques. Up to four components, monomer, initiator, porogen and crosslinker can be simultaneously introduced into our reactor. Specifying the molar flowrates of all four components enables control of solid-state free-radical polymerization occurring on the cooled substrate. A variety of films were grown under different operating conditions. Pore size, porosity, and polymer morphology are influenced by changing the component feed ratio, the pressure, and the substrate temperature. Evidence of polymerization-induced phase separation (PIPS) and the control over porosity and morphology will be discussed in detail using data from GPC, optical, and electron microscopy.